This invention relates to apparatus for cooling hot steel plate and sheet, and more particularly to apparatus for rapidly cooling hot steel plate and sheet while preventing distortion of the plate or sheet.
As is well known, in rolling steel plate and sheet, such as in a plate-mill plant, rolled hot plates are generally flattened in a hot leveler, allowed to cool naturally in the air while traveling over cooling beds, and are delivered to shearing and inspection lines. This air-cooling method requires several hours for cooling the as-rolled plate to approximately ambient temperature, though the time required varies somewhat with the initial or finishing temperature, plate thickness and cooling conditions.
The productivity of modern plate mills has been improved so much that each mill manufactures 200,000 to 300,000 tons of plate per month. To handle such large tonnages by the conventional cooling method necessitates cooling beds having a huge area. The time from rolling to shearing becomes prolonged. Further, since the cooling time is not controllable, it becomes difficult to quickly match the flow of plates to the line speeds of the subsequent shearing and inspection sections. All these factors have caused the productivity of the entire plate-mill plant to remain lower than is desirable.
Recently, the noise produced by the cooling beds and scratches on the plates caused by the cooling beds have also been presenting problems. A change from a grid type conveying system to a walking-beam type system has increased the cost of the cooling bed. As a consequence, it has become the practice in recent years to first air-cool plates to approximately 600.degree. C., the temperature below which the properties of the steel will not change even if it is subjected to rapid cooling, on cooling beds, then cool them to a lower temperature by use of a suitable coolant. But no full-fledged equipment for this process has been provided. However, roller-press quenching equipment, conventionally used for quenching steel plates, is similar to rapid cooling equipment for cooling steel plate. This roller-press quenching equipment comprises oppositely disposed top and bottom rollers and nozzles oppositely disposed between the rollers for spraying cooling water. This equipment is normally directly connected with a reheating furnace to quench the reheated plates. It is capable of treating only relatively well-shaped plates, has a low productivity, and is unsuitable for an in-line cooling unit because of the following drawbacks.
In the conventional quenching equipment, the top pressing rollers are all attached to a top frame so that they move up and down in conjunction therewith. Therefore, if a plate having a distortion at its head or trailing end enters the unit, the plate may cause the frame to tilt and thus cause some of the rollers to separate from the top surface of the plate. As a result, the thickness and, therefore, the temperature of a layer of cooling water on the plate becomes uneven. This impairs the control of cooling rates between the top and bottom plate surfaces, and makes it impossible to produce plates of satisfactory shape. In addition, the fixed frame of this equipment, which carries a roller-height adjusting mechanism, integrally contains the top frame that is moved up and down as well as the rollers attached to the top frame. It is therefore practically impossible to detach the top roll unit from the rest of the machine. Accordingly, a serviceman has to slip between the top and bottom rollers for maintaining and repairing the rollers and nozzles. Likewise, roller and nozzle changing operations are inefficient since the rollers etc. can be disassembled from the remainder of the apparatus and these parts reassembled with the apparatus only sideways.
As long as this equipment is used off line and directly connected with the reheating furnace, its operating and failure rates remain low, and the aforementioned problems are not very acute. Lately, however, direct quenching equipment for directly treating rolled plates and cooling equipment replacing the cooling bed have been put into practical use. These uses, however, involve bent materials, temperature variations and severe operating conditions. These systems are required to have a high operating speed and productivity so as to match well with the capacity of modern rolling mills. The operating rate thereof is also increased greatly. Therefore, high reliability and maintainability are essential. Particularly the in-line type equipment is subjected to preventive maintenance that comprises regular disassembly and checking. Therefore, the conventional equipment which is difficult to disassemble is not well suited for in-line use.
Further, the in-line equipment must be designed so as to permit the threading therethrough of plates which require no cooling. It is then preferable to lift the top rollers to a given height and keep them at the elevated position, from the standpoint of vibration and noise control and energy saving. In such a case, the bottom rolls, which are used more, are apt to wear faster than the top rolls. In the conventional equipment, in which the plate is restrained from top and bottom, plate scratches due to the speed difference between the top and bottom rolls and roll wear due to slippage relative to the plate are furthered. Also, in such equipment, in which the mechanically interlocked top and bottom rolls are driven, it is difficult to stop only the top rolls or adjust the speed difference between the top and bottom rolls.
The conventional roller-press quenching equipment has a mechanism for pressing the plate to restrict its distortion during cooling to a certain extent. The pressure is adjusted by the amount of compression of a spring interposed between a screwdown device and a support frame. The pressure is established, for instance, by the amount by which the roll is raised when the plate enters the space between the rolls, i.e. the amount of spring compression, and the spring constant, the roll spacing being set at a dimension smaller than the thickness of the plate being treated. One such unit is designed to produce a maximum pressure of 110 tons when the roll rises 18mm, a value set in relation to the plate thickness. As will be understood, great pressure will not be obtained unless the roll spacing is made sufficiently smaller than the plate thickness. But a decreased roll spacing tends to inhibit the entry of the plate. From the standpoint of restraining pressure, the spring cannot be compressed too much. Therefore, if any extraordinary plate distortion occurs during cooling, an excess load is imposed on the rolls and other parts of the apparatus because the amount of rise of the rolls, i.e. the amount of compression of the springs, is limited. As a consequence, bending or breakage of the rolls may occur.
When roller-press quenching equipment is used for its original purpose of quenching, reheated plates cut to specified sizes are fed thereto. Even if the plate width changes, therefore, the center line of the quenching equipment always serves as a reference line for feeding of the plates through the equipment, thus permitting application of uniform pressure throughout the entire plate width.
But when the equipment is used as an in-line forced cooling unit as a substitute for a cooling bed in the low temperature region of the cooling of rolled plate, it is placed in a suitable position preceded by a cooling bed and generally followed by shearing equipment. Accordingly, the edge of the plate must be used as the feeding reference line. However, the plate width varies extensively; sometimes plates narrower than a half of the length of the restraining roll are treated. When pressing the plate with the restraining rolls, the roll spacing usually is set somewhat smaller than the plate thickness, and the pressing force is applied by way of an elastic body, e.g. a spring in conventional roller-press quenching equipment. When the plate enters the space between the rolls, therefore, the top roll rises to make the spacing equal to the plate thickness. In the conventional spring-supported roller-press quenching equipment, the pressing force at the opposite ends of the rolls is set to be equal. Therefore, when feeding narrower plates, especially those having widths which are smaller than a half of the roll length, the top roll is inclined around the edge of the plate near the middle of the roll, and the roll is incapable of imposing a uniform appropriate pressure throughout the entire width of the plate. When the roll is inclined, a gap occurs between the top surface of the plate and the bottom suface of the roll. Further, this gap changes over the width of the plate, which in turn varies the cooling water flow and water-film discharging conditions in the direction of the width of the plate and, thereby, prevents uniform cooling. Such uneven cooling may result in undesirable plate deformation. In the as-rolled plates having inherent deformation, this phenomenon becomes especially pronounced.
To solve this problem, either the plate or the cooling equipment must be shifted so that the plate is always passed along the center of the cooling equipment. But such equipment will naturally be costly.
For convenience of maintenance, it is also desirable that the in-line cooling equipment be so constructed as to permit the disassembly of the top frame with ease. Or it is preferably made up of a plurality of units that can be readily replaced. The restricting force, however, is imposed mainly by the weight of the top roll unit elevating member. It is therefore very difficult to provide a restraining force adjusting device in such a position that a uniform restraining force can be imposed on the front and rear and the right and left of the plate, i.e. over the entire area of the plate, due to the size of the roll unit, the position of the drive unit and cooling piping, and other reasons. If the widthwise restraining force becomes non-uniform, the plate may walk out of the cooling water spray range, thus causing unbalanced cooling, thus producing deformed plates.
As is evident from the above description, there has been no high-speed cooling equipment that can be installed in line with a plate-rolling mill. Also, utilization of the conventional roller-press quenching equipment involves the aforementioned problems.